For a long time, surface-treated steel sheet comprised of galvanized steel sheet treated on its surface by chromate has been used in large quantities for industrial products in a broad range of fields. This galvanized steel sheet had a high ability to suppress the formation of white rust which occurred when used under the usual atmospheric environment and further had the features of easily securing conductivity between the electronic board and steel member and having superior groundability and shielding performance. The ability to suppress white rust was high-due to, it was believed, the high ability of passivation of a chromate film to plated zinc material and the high ability of the film of self repair of damages. Further, the conductivity was good since the chromate treated layer was thin and uniform, so the contact resistance with the conductive terminals was kept low.
In recent years, the demands on materials for reduction of environmental load substances and toxic substances have grown tougher. There is an increasing movement toward restricting use of the hexavalent chrome used for chromate films. Hexavalent chrome is a toxic substance identified as being carcinogenic. Restrictions on discharge of hexavalent chrome in the process of production of surface-treated steel sheet and harm to health accompanying elution thereof at the time of use of the steel sheet are concerns.
Therefore, the inventors developed a treated film not using chromate at all (for example, see JP 2000-319787 A). JP 2000-319787 A disclosed the technology of coating the surface of a galvanized steel sheet with a rust preventive coating layer. To improve the corrosion resistance, phosphoric acid or an inhibitor ingredient was suitably added. This treated steel sheet was superior in corrosion resistance due to being coated with an insulating resin layer, but had the defect of being inferior in surface conductivity. Therefore, the rust preventive steel sheet of JP 2000-319787 A cannot at present be said to have sufficient characteristics for application to home electronics, office automation products, and other equipment stressing groundability.
Here, “groundability” means making the potential of the surface of the steel sheet caused by electromagnetic waves emitted from electronic components or electromagnetic waves coming from outside the device the same as the ground potential. If this groundability is insufficient, trouble such as erroneous operation or breakdown of the electronic equipment, noise, etc. will occur.
Electronic equipment up to now have generally secured this groundability by being screwed to steel housings, chassis, etc. In this case, the end faces of the steel sheet were exposed at the screw holes, so metal-metal conduction could be easily obtained regardless of any chromate layer. However, along with the increasingly smaller size and higher performance of electronic equipment in recent years, the number of complicatedly shaped parts has increased, the screw-fastened parts have decreased, and parts have increasingly been joined by contact of steel sheet surfaces, or contact by caulking or leaf springs. In this case, it is important that the surface of the plated steel sheet be small in contact resistance. In systems coated with insulating resin explained above, the groundability therefore became insufficient.
As prior art for improving this groundability, JP 2004-277876 A forms an intermediate layer having groundability at the surface of the plating layer and further forms an organic resin layer on the surface thereof and specifies that the coverage rate of the organic resin layer is at least 80% and for the surface roughness of the steel sheet, an arithmetic average roughness Ra is 1.0 to 2.0 μm and a filtered center line waviness Wca is not more than 0.8 μm.
Further, JP 2005-238535 A discloses the art of obtaining a surface roughness of a sheet stock to be plated by defining the surface roughness Ra and PPI of electrodischarge treated temper rolling rolls and securing conductivity of the resultant obtained galvanized steel sheet without impairing the corrosion resistance.
Furthermore, JP 2002-363766 A defines the surface roughness of the sheet stock itself to be plated by the count of peaks and the Ra so as to achieve both corrosion resistance and conductivity.
However, while JP 2004-277876 A, JP 2005-238535 A, and JP 2002-363766 A all demonstrate an effect of improvement of the conductivity, the performance is not stably expressed and depending on the production line, conductivity cannot be secured. Development of technology stably securing conductivity has been desired.
Galvanized steel sheet coated with a chromate-free film is produced by subjecting a coil-shaped steel sheet continuously to a plating treatment and chromate-free treatment. The plating method includes electroplating and hot dip plating. The former is the art of electrochemically causing the precipitation of zinc in an aqueous solution containing Zn ions, while the latter is the art of immersing a steel sheet in a molten state metal zinc bath to form a film. The surface configuration of the plating is, in the case of electroplating, high in uniformity of formation of the plating, so the surface configuration of the sheet stock is maintained, but with hot dip plating, the leveling property is high and configuration is generally imparted by transfer of the configuration of the temper rolling roll after plating. The plated steel sheet is coated with a resin-based or inorganic chromate-free film or chromate film, baked, and dried by a later post-treatment section. After this, it is coiled to obtain the final product.